Lecture 16 Channel Cross Sections USU （16通道横截面通常的讲座）.pdfVIP

Lecture 16 Channel Cross Sections I. Channel Cross Section Parameters • Common cross-sectional shapes are rectangular, trapezoidal and circular • Geometrically, rectangular cross-sections are just special cases of trapezoidal sections • Circular cross sections are hydraulically more “efficient” than other cross- sectional shapes, but they are only used for small channel sizes • Circular cross sections are usually made of precast concrete mixes, and elevated above the ground • Tunnels designed for open-channel flow are sometimes built with special cross sections (e.g. “horseshoe” sections) • The standard horseshoe cross section has a semicircular top portion, and an intersection of three larger circles for the lower portion – it can be considered a modification of a circular section • A semi-circular channel cross section is the best shape for an open-channel, including open-channel flow in tunnels, but the horseshoe shape has been used in dozens of tunnels to allow for greater floor width, thereby facilitating the passage of equipment through the tunnel Standard horseshoe cross section (bold curves): diameter of upper section is half of the diameter of the three larger circles BIE 5300/6300 Lectures 171 Gary P. Merkley • Trapezoidal cross sections can be symmetrical or non-symmetrical • For trapezoidal cross sections, the inverse side slope (H:V) is usually between zero and 2.0 • Common inverse side slopes are zero (rectangular section), 1.0 and 1.5 • There are tradeoffs between low and high values of side slope: 1. canals with low inverse side slopes occupy less land area 2. high inverse side slopes are more stable and may require less maintenance 3. high inverse side slopes are safer,